1. Field of the invention
This invention relates to a display system used in computer systems or the like which displays characters on a display, e.g. CRT or LCD.
2. Prior art
Conventional display systems include character pattern ROM, display data RAM, a display controller (CRT controller) and a display device (CRT). The character pattern ROM contains pattern data of a plurality of characters each of which corresponds to a character code. When a character code is input into the character pattern ROM, the corresponding character pattern is output from the character pattern ROM to the display device via a display signal processor. The display data RAM stores a plurality of character codes constituting the display screen which is to be displayed on the display device. The data in the display data RAM can be changed to produce a change in the display screen. The display controller outputs an address of the display data RAM to the display data RAM, which outputs a character code at the address to the character pattern ROM to be displayed on the display device.
For outputting specific character pattern data from the ROM, the RAM should be constituted so as to be able to specify each character pattern data stored in the ROM. That means, the display data RAM should store and output data that can discriminate between the many character pattern data stored in the ROM. For example, if the ROM has a capacity of 32 kbit (4 k x 8 bit), 12 bit data is necessary to address each character pattern of the character pattern ROM. If a display data RAM of 8 bit output is employed, two display data RAMs are necessary at one time to address the character pattern ROM.
Therefore, when the number of pattern data stored in the character pattern ROM is increased and the number of bits needed to address the character pattern ROM is accordingly increased, the necessary number of display data RAMs is also increased. This results in complicated circuitry and a circuit board of large area. Further, such results lead to an increased vulnerability of the circuit board to static electric noise and AC line noise.
Another display system includes a RAM that stores attribute information (such as underline) besides character codes. In this system, the pattern data outputted from the ROM is combined with the attribute to be displayed on the display device. In such a system, the number of bits outputted from the RAM is further increased and more RAM's are accordingly required.